Manufacture of a filtering screen

ABSTRACT

In a method of and apparatus for manufacturing a filtering screen, a frame of plastics coated metal or reinforced plastics material is located in a jig and at least two sheets of wire cloths are placed over the frame and their edges gripped by pneumatically operated clamps carried by the jig. The clamps are pneumatically driven outwardly of the frame to tension each cloth in orthogonal directions. The tensioned cloths are pressed against the frame and heated in order to bond the cloths to the frame. The heated material is allowed to cool, and after completion of the bonding step the clamps are released, allowing the frame to be removed from the jig. Where necessary the cloths are trimmed back to the edges of the frame. A separate set of pneumatically operated clamps may be provided for gripping all four edges of each wire cloth layer to allow each layer to be individually tensioned.

FIELD OF THE INVENTION

This invention relates to a method of manufacturing a filter screen,apparatus for manufacturing a filter screen and a filter screen whenmanufactured by said method and/or apparatus. The invention isespecially concerned with filter screens for use as filters in vibratingfiltration equipment such as shakers which are used in the oil drillingindustry for separating solids from the liquid phase of oil and waterbased muds retrieved from drilling operations.

PRIOR ART

An example of such a filter screen is described in Patent SpecificationNo WO95/23655. This screen has upper and lower wire mesh cloths ofdiffering qualities stretched over a metal rod reinforced frame ofplastics material to which the cloths, after tensioning, are bonded byadhesive. Later Patent Specification No. WO98/37988 describes a frame towhich the tensioned cloths can be bonded by heat softening the surfaceof the frame and pressing relevant parts of the cloths into the softenedplastics material, the bond being completed by then allowing theplastics material to cool and cure.

THE INVENTION

According to one aspect of the invention there is provided a method ofmanufacturing a filtering screen according to which a frame of plasticscoated metal or reinforced plastics material is located in a jig, atleast one wire mesh cloth is placed over the frame and its edges aregripped by pneumatically operated clamps carried by the jig, the clampsare pneumatically driven outwardly of the frame to tension the cloth inorthogonal directions, the tensioned cloth is pressed against the frameby a heated platen in order to bond the cloth to the frame and aftercompletion of the bonding step the clamps are released, the frame isremoved from the jig and where necessary the cloth trimmed back to theedges of the frame.

According to another aspect of the invention, there is providedapparatus for manufacturing a filtering screen comprising a jig forlocating at least one frame of plastics coated metal or reinforcedplastics material, pneumatically operated clamps carried by the jig forgripping the edges of a metal wire mesh cloth placed over the at leastone frame, pneumatically driven tensioning means on the jig for drivingthe clamps outwardly from the frame to tension the cloth in orthogonaldirections, a platen, means for heating the platen, means for drivingthe heated platen and/or the jig to press the cloth against the frame inorder to locally melt the plastics material and allow the cloth tobecome embedded therein, and then to retract the platen and/or jig toallow the at least one frame and cloth to cool in order to bond thecloth to the frame, and to allow the frame then to be removed from thejig after release of the clamps.

The invention also relates to a filter screen manufactured by theabove-described method and/or apparatus.

Preferably two or sometimes three layers of wire mesh cloth are bondedto the frame.

Typically a lower cloth of coarser mesh is located below a finer meshcloth.

The plastics material of the frame is typically polypropylene.

The frame itself preferably has a rectangular shaped periphery with alattice of intersecting bars. During the heating step, the plasticsmaterial forming the upper surface of the frame is softened and thetensioned cloths are pressed into the softened plastics material andbecome embedded therein. The bond is completed by allowing the frame tocool and the plastics material to cure. It is possible for the uppersurface of the frame to be ridged to aid the bonding step, in that it isthen only necessary for the heated platen used to press the clothsagainst the frame, to soften the ridges which then accept the wire meshto form the bond.

It is most desirable to protect the cloth(s) from direct contact withthe heated platen, and for this purpose a heat resistant non-stickfabric, such as PTFE glass fabric, is preferably laid over the clothsprior to the bonding step. The fabric is removed on completion of thebonding step.

When two, or occasionally possibly even three wire mesh cloths are to beincorporated into the screen, a separate set of pneumatically operatedclamps is provided for each cloth. Similarly, a cloth tensioningmechanism is provided for the clamps along at least two adjacent sides,and preferably all four sides, of each cloth. The cloths can thus beindividually and differentially tensioned as appropriate for the use towhich the screen is to be put, prior to being collectively bonded to theframe.

Preferably, each clamp comprises a fixed pair of jaws and an inflatableenvelope between them for use in gripping the edge of the wire meshcloth. Thus the envelope, when inflated, may act to press a metal,typically aluminium, plate towards one of the fixed jaws, which ispreferably lined with a rubber (or like material) strip, typically 3 cmwide, to improve the grip on the edge of the cloth sandwiched betweenthe plate and the jaw.

A control panel is provided to enable a jig operator to activate thepneumatic clamps and the pneumatic tensioning means. Preferably fourcontrol panels are provided, so that the operator can feed the clothinto the clamps along one side of the frame and activate these clamps toclamp that edge of the cloth using one control panel, before moving toanother side of the fame and repeating the procedure for that side ofthe frame, and so on. Where there are two or more levels of clamps andtensioning mechanisms, one level for each layer of cloth, the firstcloth is individually clamped and tensioned on all four sides, and thenthe next, in a similar manner until all the cloths have been clamped andtensioned as appropriate.

According to a preferred feature of the invention the clamping andtensioning of the cloths is performed at a first station remote from asecond station at which the bonding occurs. Pneumatic power for theclamps and tensioning mechanisms is therefore supplied to the jigthrough a flexible supply cable, in order to allow for the movement ofthe jig between the stations.

Initially, therefore, the at least one frame is loaded into a jigcarried by a cradle which, after the cloths have been clamped andtensioned, is conveyed to the second station containing the heatedplaten.

The cradle preferably has wheels running on rails along which the cradleis driven between the first and second stations by a suitable drivemechanism, conveniently a Festo® linear pneumatic drive.

The jig cradle and the platen, which is preferably pre-heated, are thenmoved relatively vertically.

In one embodiment, the jig cradle is raised up to the platen by ahydraulic ram, to commence the bonding step. Thus, the ram may actagainst the underside of the cradle to lift it off the rails on whichthe cradle is driven between the first and second stations.

Where a non-stick fabric is to be placed on top of the cloth before itis engaged with the heated platen, the fabric is preferably placed onthe cloth at the first station, although if preferred it can be added atthe second station, or in transit.

Conveniently the platen may be pre-heated to a temperature in the range200 to 300 degrees C., preferably about 250 degrees C.

Where a hydraulic ram is employed to elevate the cradle and then squeezethe mesh between the heated platen and the frame, a squeezing force istypically obtained using a hydraulic pressure applied to the ram in therange 500 to 2000 psi.

Preferably this hydraulic pressure is adjustable, for example to suitthe screen materials, and in particular the cloth or cloth combinationsused.

The hydraulic pressure to effect the squeeze and heating and bondingstep is typically applied for a period of time in the range 30 secondsto 2 minutes.

At the end of the heating and bonding step, the jig cradle and platenare separated to allow the material to cool and cure, and finally thejig cradle is conveyed back from the second station to the clamping andtensioning station, to allow the operator to release the clamps, removethe protective fabric, remove the frame or frames from the jig cradle,and trim the cloths extending beyond the edges of the frame(s).

Other features which may be incorporated into the above-described methodand apparatus are as follows.

Firstly, the clamping surfaces of the clamps may be sufficiently roughto prevent cloth slipping when the cloths are put under tension butsmooth enough to prevent tearing of the cloths.

Secondly, a preferred method and apparatus handles two frames at a time,arranged side-by-side, and a single large area piece of wire mesh cloth,one for each layer, is stretched over both frames. After the bondingstep the single piece of cloth is cut along a line between the twoside-by-side frames. It will be appreciated that the jig cradle is thusrelatively large, but the time taken to fit, clamp and tension eachlarge sheet is little different from what would be required to similarlyhandle smaller areas of cloth over a single frame, and each heating andbonding step results in two screens instead of just one.

Thirdly, the clamps of any one layer can preferably be moved outwardlyof the frame individually and independently of each other, i.e. eachclamp has an associated tensioning mechanism, so that any localisedslack in the cloth can be taken up.

Fourthly, a breaker bar may be located immediately in front of each lineof clamps along the sides of the frame, and the cloth moves over it asthe cloths are tensioned, thereby to remove any ripples in the cloth.

DESCRIPTION OF EMBODIMENT

The invention is further described with reference to the accompanyingdrawings, in which:—

FIG. 1 is a side elevational view of the filter screen manufacturingapparatus,

FIG. 2 is a side view showing some internal detail of the apparatus whenseen from a vertical longitudinal centre plane therethrough,

FIG. 3 is an elevational view on the other side of the apparatus,

FIG. 4 is an end view of the apparatus,

FIG. 5 is a plan view of the apparatus, the jig being shown with a firstframe located therein,

FIG. 6 is an enlarged view of a longer side of the jig seen when viewingthe end of the apparatus,

FIG. 7 is an enlarged view of the frame to be located in the jig,

FIG. 8 is an enlarged view of the jig with the first frame locatedtherein,

FIG. 9 is a view similar to that of FIG. 8 but after the two frames havebeen located in the jig, a wire mesh cloth, cut away in part, tensionedthereover, and a release paper, also cut away in part, laid over thecloth,

FIG. 10 is a side view, further enlarged, of the pneumatic clamps on thejig, and FIGS. 10A and 10B illustrate the three sets of clamps along oneside of the jig, ready to receive cloth edges (in 10A) and after beingfully retracted (in 10B), such as when tensioning cloths (not shown)

FIG. 11 shows the pneumatic connections for the clamps on the jig,

FIGS. 12, 13 and 14 are diagrams showing the pneumatics for the clothclamping and tensioning system of the apparatus,

FIG. 15 is a diagram showing the electrical supply circuits of theapparatus, and

FIGS. 16 and 16A show control panels for the apparatus.

Referring to FIGS. 1 to 6, the illustrated apparatus comprises asupporting framework 10 which at one end supports a cradle 12 forming ajig for enabling assembly of screen frames at an assembly station 14,and which at the other end is integrated with a heating platen supportunit 16 constituting a bonding station 18. The cradle 12 has wheelswhich run on parallel spaced apart rails 22, 22A to enable the cradle tobe moved between the two stations by a drive 23 (see FIG. 3). At leastone rail and the wheels corresponding therewith are designed to ensurethat the cradle moves in a straight path.

The drive includes a pin 25 which engages in a groove 27 in plate 29,which latter is secured to and extends from the cradle (see FIG. 3). Thedrive 23 is a Festo® linear pneumatic drive.

A flexible services umbilical cord 24 conveys electrics and pneumaticsto the cradle.

At the screen assembly station, two GRP screen support frames 26, one ofwhich is shown in FIG. 7, are laid side by side in the jig cradle 12.FIG. 8 shows one of the frames 26 located in the cradle 12, with thespace for a second frame 26 beside it. The plan view of the apparatus(FIG. 5) also shows one frame located in the jig. Each frame 26comprises a rectilinear grid of orthogonally intersecting, wirereinforced glass fibre reinforced polypropylene bars as described inPatent Specifications WO95/23655 and WO98/37988. In known manner, theupper surfaces of the peripheral edges and intersecting bars of theframe are moulded with upstanding ridges.

Having located the two frames in the jig woven wire mesh filter clothsare laid in turn over the frames, and the edges of each cloth insertedinto and gripped by pneumatic clamp units 28 carried by the jig beforethe next cloth is applied. The pneumatic clamp units are shown in moredetail in FIG. 10.

Each clamp comprises fixed upper and lower jaws 28A, 28B the lower ofwhich carries on its upwardly facing surface an inflatable envelope 28Cover which, lies a protective aluminium plate 28D. A cloth edge isinserted between 28D and a rubber strip 28E on the underside surface ofthe upper jaw, so that when 28C is inflated the cloth is clamped betweenplate 28D and the upper jaw, the rubber strip 28E reducing the risk ofslip occurring when the cloth is tensioned. In use, the edge of a wirecloth is pulled over the breaker bar 31 and inserted between the jaws28A, 28B between the rubber strip and the aluminium plate. The envelopeis then inflated to grip the cloth between the rubber strip and theplate with a pressure sufficient to withstand the tensioning forcessubsequently to be applied to the cloth.

As best seen in FIGS. 10A and 10B the rear edge of each aluminium plate28D is upturned to form a lip which engages in a groove in the undersideof the upper jaw 28A near the rear edge thereof.

In order to enable cloth tensioning, jaws 28A, 28B are joined at theirrear to form a single assembly carried by the piston 28F of a doubleacting pneumatic cylinder 28G. The double acting pneumatic cylinder andpiston 28G, 28F constitute a tensioning device and there is one suchdevice for each clamp unit. Supplying air to the forward end of thecylinder 28G forces the piston rearwardly, and thereby the jaw assembly,back away from the jig to stretch the gripped cloth over the breaker bar32 at the periphery of the jig. Supplying air to the rear end of thecylinder moves the jaw assembly forward to its rest position nearer thejig as shown in FIG. 10A. This is effected after the cloth has beenbonded to the frame and the jaws released, as later described. Thebreaker bar 32 prevents any local rippling of the wire cloth duringtensioning.

In practice, it will be understood that all four edges of a wire clothare gripped prior to tensioning, as will be clear from laterdescription.

The pneumatic supply for the clamp units is later described withreference to FIG. 11.

Three layers of clamp units 28 are provided, to allow up to three clothsto be laid over and secured to the frames.

Three lines of clamp units 28 (such as can be seen in FIGS. 10, 10A and10B) are provided on all four sides of the cradle, and a master controlpanel 30 (see FIGS. 5 & 6) is provided at one corner, and three slavecontrol panels 30A, 30B and 30C are located at the other corners, sothat an operator can control the clamps and tensioning devices alongsuccessive sides of the cradle individually, by simply moving from oneside to the next around the assembly station.

Having gripped all four edges of a wire mesh cloth between the clampunit jaws, the latter are pneumatically driven outwardly of the jig bymeans of the aforesaid cylinder and piston devices, in order to tensionthe cloth over the two frames. Each tensioning device, constituted bythe aforesaid double acting pneumatic cylinder and piston, is operableindependently, to stretch the cloth over the breaker bars 32 extendingalong the four edges of the jig cradle, whereby to ensure that the clothis uniformly tensioned without ripples or creases.

The process is repeated for each layer of cloth, the edges of the firstcloth being located in the jaws of the lowest of the three lines ofclamp units 28, the edges if the next layer of cloth in the next line ofclamp units, and so on.

The complete cradle can then be moved to the bonding station, under theheating platen on the unit 16, best seen in FIGS. 2 and 5. First,however, before bonding, in order to prevent direct contact between theupper screen cloth and the heating platen, a heat resistant non-stickfabric, e.g. PTFE glass fabric, is laid over the tensioned cloth in thejig cradle. If desired the edges can be gripped by a line of clampunits. FIG. 9 shows the jig 12 with the two frames 26 located therein,upper and lower wire mesh cloths 26A and 26B cut away in part, stretchedover the frame, and the release fabric 26C, also cut away in part, laidover the cloths.

At the bonding station, the jig is raised and lowered by a platform 31acted on by a hydraulic ram 34, the platform 31 engaging the undersideof the jig 12 to lift it, and in so doing lifting the wheels 33, 35 offthe rails 22, 22A. On being raised the cloth on the screens is broughtinto contact with the platen 36, which is pre-heated by an electricheating element (not shown).

The heated platen softens the ridges on the upper edges of theinterstices and the side flanges of the plastics frames, and the warpand weft wires of the tensioned cloths are pressed into the softenedmaterial. The ridging of the side flanges, and interstices aids thebonding process, as described in the aforementioned publishedInternational Patent Specifications.

After bonding the screen assembly is then lowered and moved back to theassembly station.

Back at the assembly station, the PTFE release fabric is stripped off,the tension on the cloths is released, the screen cloths are cut betweenthe two frames, each frame is removed from the jig, and the protrudingedges of the wireloth are trimmed back to the edges of the frames by forexample using an angle grinder. Manufacture of the screens is thencomplete, and the jig is now ready to receive the next two frames andlayers of cloth to make the next two screens.

For completeness, FIGS. 11A and 11B show the air supply lines to theclamp units. Air for inflating the envelope 28C in the jaws 28A, 28B issupplied through air line 40. Complete evacuation of the envelope isrequired fully to open the space between the jaws prior to insertion ofthe edges of a fresh wire mesh cloth and this is achieved by applyingvacuum to line 40.

Air for operation of the double acting cylinder 28G is supplied throughair lines 42, 44. Supplying air through line 42 drives the piston 28F,and thus the clamp, outwardly of the jig in order to tension a wire meshcloth clamped between the jaws. Supplying air through line 44 drives theclamp back towards the jig at the end of the operational sequence, sothat the clamps are back in the positions shown in FIG. 10A ready toreceive the next screen cloths for bonding to the next pair of frames.

The pneumatic control circuit for the clamps and tensioning devices isshown in FIG. 12.

Air under pressure is supplied via an on/off valve 46 to a pressureadjustable valve 48 and associated pressure meter 48A and thence througha filter and valve unit 49 to a pressure line 50 from which air can besupplied via a pneumatic switching circuit 52 to operate the clamps andtensioning devices. The switching circuit is controlled by buttons onthe control panels at the corners of the jog, those buttons beingrespectively labelled L3, L2, L1 and P1 in the drawing. Each linepressure for the clamp and tensioning devices is adjustable by means ofpressure adjusters and associated meters 54. A typical line pressure P1for operating the clamps may be 5.5 bar, and typical tensioningpressures may be 4.8 bar (L3) for the uppermost layer of tensioningdevices, 3.0 bar (L2) for the intermediate layer and 2.0 bar (L1) forthe bottom layer. There may, of course, be only one or two wire meshcloths applied in the manufacture of any particular screen, in whichcase some of the buttons for the unused layers of tensioning deviceswill not be operated. The exemplary line pressures mentioned for thetensioning devices are given assuming all three layers of tensioningdevices are to be used. The valves employed in the pneumatic controlcircuit are all solenoid valves.

FIGS. 13 and 14 show the set up of the pneumatics for the clamp units.First, it has to be understood that the frames to which the wire meshcloths are to be bonded are rectangular, with two shorter sides A and Band two longer sides C and D. The two frames are located in the jig withtheir shorter sides A and B in line, thus requiring six clamp unitsalong each of the aligned edges, (twelve in all) for a cloth laid overboth frames. Four clamp units are provided along each longer side of theframes at opposite ends of the jig (eight in all).

FIG. 13 shows the set up for six clamp units along one side of the jig;this set up is repeated for the other side.

FIG. 14 shows the set up for the four clamp units along the longer endsof the jig. The set up of FIG. 14 is also repeated at the opposite end.In both figures, the set up is shown for the clamp units in all threelayers.

In both of FIGS. 13 and 14 switching valves 56 control the inflation andcollapse of the envelopes in the clamps through lines 58 and 60. Ventingthe envelopes to atmosphere via 60 releases the cloths to allow theiredges to be removed. Applying vacuum to 60 by using 62, collapses theenvelopes to facilitate the insertion of new cloth between the clampingjaws.

Valve units 64 control the supply of air to the pneumatic cylindertensioning devices through lines 64, 65, line 64 for tensioning and line65 for untensioning.

In FIG. 15, reference 68 indicates the fused electrical power inputterminals; reference 69 indicates the power supply to the motor drivingan oil pump for supplying hydraulic oil under pressure to the ram whichlifts and lowers the cradle at the bonding station; reference 70indicates the power supply to a photoelectric safety circuit which isprovided to stop and reverse the hydraulic drive if a light beam in thepath of the cradle across the entrance to the bonding station isinterrupted for any reason while the cradle is being lifted towards theheating platen; reference 71 indicates the power supply enablingcomputer control of the sequence of the operating procedure of theapparatus; reference 72 indicates the power supply for all AC solenoidvalves employed in the pneumatic circuits and reference 74 is the powersupply for all DC solenoid valves, and reference 76 indicates the powersupply to the heater used in heating the platen at the bonding station.

Finally FIGS. 16 and 16A respectively show the master control panel 30at one corner of the jig, and one of the secondary (or slave) controlpanels 30A, 30B, 30C at one of the other corners of the jig. The buttonsare marked with their functions.

The main panel 30 and each secondary panel 30A, 30B, 30C provides forclamping, unclamping and evacuation of the clamp envelops, along thefirst, second, third and fourth sides of the jig respectively, ifappropriate for all three layers of cloth.

The main control panel 30 additionally provides for tensioning anduntensioning along all four sides of the jig, including the first side,which is where the operator starts the loading of the jig. Thus, theoperator, having first located the edge of a cloth in the clamps alongthe first side of the jig, uses the main control panel 30 to operatethose clamps to clamp that edge, and then moves round the jig to clampthe edges of the cloth in sequence on the second, third and fourthsides. This brings the operator back to the main control panel, at whichthe appropriate tensioning button can be pressed to operate all thepneumatic cylinder tensioning devices simultaneously so all the clampsfor one layer are moved outwardly at the same time, thus tensioning thecloth appertaining to that layer.

When loading has been completed and the cradle moved back to theassembly station, the operator can use the main control panel 30 firstto apply pressure to reverse all the tensioning pneumatic cylinders ofall the cloth layers.

All the clamping can be released by evacuating all the inflatedenvelopes to atmosphere, and the latter can be flattened to assist ininserting more wire cloth edges, by applying vacuum to all theenvelopes.

It is important to note that, during tensioning, the same air pressureis applied to the pistons of the pneumatic tensioning devices and thesemay not all move exactly equally, but each will move to the extentnecessary to tension all parts of the relevant cloth to thepredetermined and preset extent.

1. Apparatus for manufacturing a filtering screen comprising a jigadapted to receive and locate at least one frame of plastics coatedmetal or reinforced plastics material, pneumatically operated clampscarried by the jig adapted in use to grip the edges of a layer of wirecloth placed over the at least one frame, pneumatically driventensioning means on the jig adapted to drive the clamps outwardly fromthe frame to tension the cloth in orthogonal directions, a platen, meansfor heating the platen, drive means adapted in use to move the heatedplaten and/or the jig thereby to press the cloth against the frame inorder to heat and locally melt the plastics material and cause the wiresof the cloth to become embedded therein, and then to retract the platenand/or jig so as to remove the source of heat from the at least oneframe to allow the frame and cloth to cool, in order to bond the clothto the frame, and to allow the frame with cloth attached thereafter tobe removed from the jig after release of the clamps, wherein each clampcomprises a fixed pair of jaws and an inflatable envelope between themfor use in gripping the edge of the wire cloth.
 2. Apparatus as claimedin claim 1 wherein the envelope is separated from the cloth by a metalplate sandwiched between the cloth and one of the fixed jaws. 3.Apparatus as claimed in claim 2 wherein the other said jaw is lined witha rubber (or like material) strip to improve the grip on the edge of thewire cloth sandwiched between the plate and the jaw.
 4. Apparatus asclaimed in claim 1 wherein a separate set of clamps and tensioning meansis provided for each layer of wire cloth adapted to grip the four edgesof each layer independently of the other layers of wire cloth, and inuse independently tension each layer of wire cloth.
 5. Apparatus asclaimed in claim 1 further comprising a control panel to enable a jigoperator to activate the pneumatic clamps and the pneumatic tensioningmeans.
 6. Apparatus as claimed in claim 1 wherein four control panelsare provided, so that the operator can feed the cloth into the clampsalong one edge of the cloth and activate these clamps to clamp that edgeof the cloth using one control panel, before moving to another edge ofthe cloth and repeating the procedure for that edge of the cloth and soon until all four edges of the cloth have been clamped, and thereaftertensioning the layer of cloth that has been so clamped.
 7. Apparatus asclaimed in claim 6 wherein there are two or more levels of clamps andtensioning mechanisms, one level for each layer of cloth, and eachcontrol panel enables the clamping and tensioning of each level to beperformed, so that a first cloth is individually clamped and tensionedon all four sides, and then the next, in a similar manner, until all thecloths have been clamped and tensioned as appropriate.
 8. Apparatus asclaimed in claim 7 comprising a first station where the clamping andtensioning is performed and a second station containing the platen wherethe bonding occurs, and wherein said jig is carried by a cradle which ismoveable between the two stations.
 9. Apparatus for manufacturing afiltering screen for use in vibrating filtration equipment, theapparatus comprising a jig adapted to receive and locate at least oneframe of plastics coated metal or reinforced plastics material,pneumatically operated clamps carried by the jig adapted in use to gripthe edges of at least two layers of wire cloth placed over the at leastone frame, pneumatically driven tensioning means on the jig adapted todrive the clamps outwardly from the frame to tension the cloth inorthogonal directions, a platen, means for heating the platen, drivemeans adapted in use to move the heated platen and/or the jig thereby topress the cloth against the frame in order to heat and locally melt theplastics material and cause the wires of the cloth to become embeddedtherein, and then to retract the platen and/or jig so as to remove thesource of heat from the at least one frame to allow the frame and clothto cool, in order to bond the cloth to the frame, and to allow the framewith cloth attached thereafter to be removed from the jig after releaseof the clamps, wherein a separate set of pneumatically operated clampsis provided for gripping all four edges of each wire cloth layer and aseparate cloth tensioning means is provided for the clamps along atleast two adjacent sides of each cloth layer, whereby in use the wirecloths are individually, and if required, differentially, tensioned as,appropriate for the use to which the screen is to be put, prior to beingcollectively bonded to the frame and wherein the apparatus comprises afirst station where the clamping and tensioning is performed and asecond station containing the platen where the bonding occurs, andwherein said jig is carried by a cradle which is movable between the twostations.
 10. Apparatus as claimed in claim 9 wherein each clampcomprises a fixed pair of jaws and an inflatable envelope between themfor use in gripping the edge of the wire cloth.
 11. Apparatus as claimedin claim 10 wherein the envelope is separated from the cloth by a metalplate sandwiched between the cloth and one of the fixed jaws. 12.Apparatus as claimed in claim 11 wherein the other said jaw is linedwith a rubber (or like material) strip to improve the grip on the edgeof the wire cloth sandwiched between the plate and the jaw. 13.Apparatus as claimed in claim 9 further comprising a control panel toenable a jig operator to activate the pneumatic clamps and the pneumatictensioning means.
 14. Apparatus as claimed in claim 9 wherein tourcontrol panels are provided, so that the operator can feed the clothinto the clamps along one edge of the cloth and activate these clamps toclamp that edge of the cloth using one control panel, before moving toanother edge of the cloth and repeating the procedure for that edge ofthe cloth and so on until all four edges of the cloth have been clamped,and thereafter tensioning the layer of cloth that has been so clamped.